1. Field of the Invention
The invention relates to an optical space communicating apparatus in which an optical transmitter and an optical receiver are assembled in the same casing.
2. Related Background Art
As a conventional optical space communicating apparatus, for instance, as shown in FIG. 1, there is known a unidirectional communication system comprising an optical transmitter 203 to which a modulation device 201 is connected through a coaxial cable 202 and an optical receiver 204 to which a demodulation device 206 is connected through a coaxial cable 205. As shown in FIG. 2, there is also known a bidirectional communication system comprising a pair of bidirectional communication apparatuses 303 and 304 in which an optical transmitter and an optical receiver are assembled in the same casing. In case of the optical space communication, since the light has a linear propagation property, the communication is generally executed in an open space which can be seen through. The optical transmitter/receiver or the optical communication apparatus is set at a perspective location which can be seen through.
In the bidirectional communication apparatuses 303 and 304, as shown in FIG. 2, modulation devices 301 and 306 and demodulation devices 302 and 305 each of which has a power supply unit are connected to the optical transmitter and optical receiver, respectively.
An example of input and output circuits of a signal and a power source in each of the above bidirectional communication apparatuses 303 and 304 will now be described with reference to FIG. 3. The bidirectional communication apparatus 303 will now be described as an example.
A power source is multiplexed to a signal from the modulation device 301 and the resultant signal is supplied to a terminal 401 through a coaxial cable 400. The modulation device 301 includes a modulator 301-1 and a power source unit 301-2. The modulator 301-1 is coupled to the terminal 401 through a high pass filter 301-3 and the power source unit 301-2 is coupled to the terminal 401 via a switch and a low pass filter 301-4. The signal from the modulator 301-1 through the high pass filter 301-3 passes through a high pass filter (HPF) 402 and a terminal 403 and is supplied to a light emitting unit 421 of the optical transmitter in the bidirectional communication apparatus 303. On the other hand, a power input signal which is supplied from the power source unit 301-2 of the modulation device 301 through the terminal 401 passes through a low pass filter (LPF) 404 and is supplied to a power distribution circuit 409. A light beam from the light emitting unit 421 is converged by a converging lens 423 and sent to another bidirectional communication apparatus 304.
A light beam from another bidirectional communication apparatus 304 is converged by a converging lens 424 and is received by a light receiving unit 422. The reception signal from the light receiving unit 422 of the optical receiver in the bidirectional communication apparatus is connected to a terminal 407 and passes through a high pass filter (HPF) 406 and a terminal 408 and is supplied to the demodulation device 302 through a coaxial cable. The demodulation device 302 includes a demodulator 302-1 and a power source unit 302-2. The demodulator 302-1 is coupled to the terminal 401 through a high pass filter 302-3 and the power source unit 302-2 is coupled to the terminal 401 via a switch and a low pass filter 302-4. The signal from the high pass filter 406 is applied to the demodulator 302-1 through the high pass filter 302-3. A power input signal from the power source unit 302-2 of the demodulation device 302 is connected to the terminal 408 through the coaxial cable 400 and passes through a low pass filter (LPF) 405 and is supplied to the power distribution circuit 409.
The power source which is supplied to the power distribution circuit 409 is supplied to the light emitting unit 421 and light receiving unit 422 through a terminal 410 and a power circuit 420. The light emitting unit 421 and light receiving unit 422 are driven by the power source.
In case of the power input/output circuit of the bidirectional communication apparatus shown in FIG. 3 mentioned above, when the power source unit of either one of the modulation device 301 and the demodulation device 302 which are connected to the terminals 401 and 408 is set to OFF, there is a possibility such that the power source current from the other apparatus flows into the apparatus whose power source unit is OFF. Therefore, as shown in FIG. 4, there is also a construction such that an isolator 411 formed by two diodes 2 is connected between the power distribution circuit 409 and the LPFs 404 and 405, thereby preventing the inflow of the power source current mentioned above. A circuit section shown in FIG. 4 is also connected in a manner similar to FIG. 3. However, the above bidirectional communication apparatus has the following problems.
In case of the bidirectional communication apparatus, it is desirable that the power in the apparatus is commonly used for the transmitting unit and the receiving unit because the power circuit in the apparatus is simplified.
In the case where the power is supplied to the bidirectional communication apparatus from the modulation device or demodulation device connected to the bidirectional communication apparatus, it is necessary for the user to select and switch the power source unit which is used on the side of the modulation device or demodulation device.
There is also considered a construction such that the power is provided for only either one of the modulation device or demodulation device. In such a case, however, when the bidirectional communication apparatus is used as a unidirectional communication for only transmission or reception, a situation such that no power is supplied to the bidirectional communication apparatus occurs.
With respect to the input/output circuit in the bidirectional communication apparatus shown in FIG. 3 mentioned above, the source voltage which is applied to the power distribution circuit 409 is not compensated so long as the kind of power source indicates the AC.
In many cases, the devices of the same specifications are used as modulation device and demodulation device which are connected to the bidirectional communication apparatus. Different signals in the same frequency band exist at the terminals 401 and 408 in the input/output circuit. Therefore, a large isolation must be provided between the terminals 401 and 408.
Inherently, in order to sufficiently obtain such an isolation, the low pass filters 404 and 405 are provided in the input/output circuit. In the case where the signal is in a wide band, however, since the signal frequency exists from a very low frequency band, it is difficult to design a low pass filter to separate the signal and the power source. Consequently, it is difficult to obtain the signal isolation in a wide band between the terminals 401 and 408. On the other hand, values and shapes of an inductor and a capacitor which construct the low pass filter increase, so that there is a case where enough attenuation cannot be obtained for the signal frequency in order to miniaturize the LPF. In the above input/output circuit, not only in the case of the circuit section shown in FIG. 3 but also in the case of the circuit section using the isolator as shown in FIG. 4, each of the diodes for a high withstanding voltage and a large current which are used in the isolator has a large capacitance across terminals. A sufficiently coarse coupling state is not derived for the signal frequency.